U.S. Pat. No. 4,990,558 describes graft-copolymer-based rigid thermoplastic compositions comprising, by weight: (a) about 60-95% by weight of the polymer components of a graft copolymer comprising about 10-65% by weight of the graft copolymer of a styrene polymer grafted onto a backbone of propylene material, and, complimentary (b) about 40-5% by weight of the polymer components of a rubber component comprising (1) about from 20-100% by weight of the rubber component of (i) at least one monoalkenyl aromatic hydrocarbon-conjugated diene block copolymer, (ii) at least one block copolymer which is a hydrogenated product of (i), or (iii) a mixture of at least one (i) block copolymer with at least one (ii) block copolymer, and (2) from about 80-0% by weight of the rubber component of an olefin copolymer rubber, e.g., EPM (ethylene-propylene monomer rubber). Optionally this formulation can contain 5-30 parts of a propylene polymer material per 100 parts of the graft copolymer and rubber component.
Modifying graft copolymers of styrene polymers on substrates of propylene polymer material by blending with a monoalkenyl aromatic hydrocarbon-conjugated diene block copolymer was found to improve the impact strength and ductility of the graft copolymers. Combinations of block copolymer and olefin copolymer rubbers gave a greater improvement in the graft copolymer's impact strength and ductility than was observed for the block copolymer alone. The examples describe thermoplastic compositions that contain block copolymers having the structure polystyrene-hydrogenated polybutadiene-polystyrene (S-EB-S) or polystyrene-hydrogenated polyisoprene (S-EP). The block copolymer is broadly defined to include S-EP-S and S-EB structures as well as branched and radial variations of the block copolymers.